The present invention relates to a sizing agent for carbon fibers and the carbon fibers treated with this agent. In particular, the present invention relates to a stable and homegeneous aqueous epoxy resin dispersion and carbon fibers treated with this dispersion and having a high flexibility and excellent moldability and physical properties.
Since, in general, an epoxy resin is widely used as a matrix resin of carbon fiber reinforced composite materials, the epoxy resin itself has been often used also as the sizing agent for carbon fibers. It was generally difficult to disperse the epoxy resin in water to form a stable and homogeneous dispersion in the prior art, since this resin is generally hydrophobic. Therefore, in an earlier stage of the industrialization of the technique of producing carbon fibers, a solution of the resin in an organic solvent such as methyl ethyl ketone was used, as such, as the sizing agent. Since, however, organic solvents are apt to cause fire hazard and are toxic, it has been eagerly demanded to provide an aqueous dispersion of the epoxy resin from the viewpoint of safety and hygiene.
As for the process for dispersing the epoxy resin in water, Japanese patent application Kokai publication No. 57-171767 proposed a process which comprises adding a diglycidyl ether/bisphenol A epoxy resin and a resin composition containing other epoxy resin which is solid at room temperature to water. Unfavorably, however, carbon fibers treated with this aqueous epoxy resin dispersion containing an epoxy resin of a viscosty as low as about 10 to 10.sup.3 poise (at 50 .degree. C.) were apt to be opened by a strong friction in the winding and unwinding steps and a molding process of a filament winding to undergo fluffing and yarn breakage to thereby impair their physical properties, because of the above epoxy resin have a viscosity of as low as about 10 to 1000 poise at 50 .degree. C. When the viscosity of this dispersion was increased, the stability of the dispersion was reduced correspondingly and uniform application of the dispersion to the carbon fibers became difficult unfavorably.
Japanese patent application Kokai publication No. 58-13781 proposed an aqueous dispersion containing a quaternary ammonium base-containing polyurethane resin and an epoxy to be used as the sizing agent for carbon fibers. However, since the sized carbon fibers with this dispersion are rather hard, soft-typed sizing agent is desired especially in a croth-making use.
U.S. Pat. No. 4,474,906 disclosed a high molecular weight completely theremoplastic polyurethane resin as the sizing agent for treating carbon fibers. However, this sizing agent has a drawback of impairing a working environment because of being in use by dissolving it an organic solvent.
Further, some of plastics reinforced by the carbon fiber treated with the sizing agent has an unsatisfactory mechanical property, because the agent is poorly compatible with a thermosetting matrix resin such as cured epoxy resin and has no reactive group with the matrix resin.